This application is a national stage filing of PCT application no. PCT/JP98/05860, filed Dec. 24, 1998.
The present invention relates to a RAKE receiver in direct sequence CDMA (DS-CDMA) transmission system that carries out multiple access in a spread spectrum system in mobile communications.
The DS-CDMA transmission system, which transmits information data modulation signals by spreading them into wideband signals using spreading codes of a processing gain (the number of chips per symbol) pg, is a communication system that assigns different spreading codes to users to enable them to communicate using the same frequency band.
FIGS. 16A and 16B show a configuration of a receiver employing sliding correlators in a conventional DS-CDMA transmission system. In the block diagrams below including FIGS. 16A and 16B, although suffixes -1, -2, . . . , -L are attached to the same reference numerals that each designate L circuits provided in correspondence to L signal passages, only the same reference numerals are used in the following description.
In the configuration as shown in FIGS. 16A and 16B, a spread modulation signal received by an antenna 101 is amplified by a low noise amplifier 103 after passing through a bandpath filter 102, and then undergoes frequency conversion into an intermediate frequency (IF) signal using a mixer 104, an oscillator 105 and a bandpass filter (BPF) 106, followed by linear amplification by an automatic gain control amplifier (AGC amplifier) 107. Subsequently, a square-law detector 108 detects the envelope of the amplitude of the received signal, and amplitude fluctuations are negatively fed back to the AGC amplifier 107 to compensate for the amplitude fluctuations caused by fading. The linearly amplified signal by the AGC amplifier 107 undergoes quadrature detection by a quadrature detector 109, resulting in a pair of baseband signals. The in-phase (I) and quadrature (Q) components are converted into digital values by A/D converters 112 and 113. Replica generators 115 generate spreading code replicas synchronized with delay times of multipath signals to be RAKE combined. Sliding correlators 114 despread using the spreading code replicas the spread modulation signal converted into the digital values. Channel estimators 116 and multipliers 117 carry out differentially coherent detection or coherent detection of the despread signals to demodulate the data. An adder 118 carries out RAKE combining of the demodulation outputs, and a deinterleaver 122 deinterleaves the output of the adder 118. A Viterbi decoder 123 decodes the output of the deinterleaver, and a data decision section or data restoring section 124 carries out hard decision to restore the received data.
In the conventional example as shown in FIGS. 16A and 16B, the absolute coherent demodulation scheme will be explained which is carried out by inserting pilot symbols into information symbols at fixed intervals. In land mobile communications, the received signal undergoes amplitude and phase fluctuations called fading because of changes in relative locations of a base station and a mobile station. In view of this, the channel estimators 116 estimate the complex envelopes, that is, the amplitude and phase fluctuations (or channels) caused by fading when the receiver carries out the coherent detection modulation. The channel estimators obtain received complex fading envelopes associated with the pilot symbols inserted into the transmitted information symbols at the fixed intervals, and then obtain the complex fading envelopes at individual information symbols between the pilot symbols. The multipliers 117 compensates for the fluctuations of the complex fading envelopes (channel fluctuations) of the individual information symbols using the values associated with the pilot symbols. The adder 118 carries out in-phase combining (RAKE combining) of multipath signals whose channel fluctuations are compensated for, thereby improving the ratio of the signal power to interference signals or thermal noise.
Selection of the multipath signals to be RAKE combined is carried out by the sliding correlators 114 which are called search fingers, in which average received signal powers of despread signals are measured at U timings in a multipath search range, and multipaths with great average received signal powers are selected. For example, when a single sliding correlator 119 is used, a correlation value (despread value) at one timing per symbol is selected so that the received signal power of the despread signal is selected at this timing. Sliding the timing of the spreading code one by one enables the power measurement to be achieved for the total of U timings.
Thus, selection of the paths to be RAKE combined requires to choose the multipath signals with great average signal power (after undergoing the fluctuations due to shadowing and relative location changes between the base station and the mobile station). On the other hand, under a land mobile communication environment, there are instantaneous fluctuations caused by Rayleigh fading. Accordingly, some multipaths may be missing from the paths to be RAKE combined because their received signal power may happen to be dropped due to the Rayleigh fading and has small signal power.
To circumvent the effect of such instantaneous fluctuations of the received power, the received signal power of the signals must be measured after averaging the Rayleigh fading fluctuations. To achieve this, the signal power measurement of the despread signals is iterated V times at U timings in the multipath search range, and delay profiles are formed from the average signal powers to select W greatest multipaths to be RAKE combined. Forming each delay profile by a single sliding correlator requires Uxc3x97V symbol time, and forming an average delay profile by f sliding correlators (search fingers) require (Uxc3x97V)/f symbol time. The timings of the spread code replicas used in the RAKE combining fingers are updated every time the delay profile is generated. When the mobile station moves fast with respect to the base station, the delay profiles fluctuate quickly. Accordingly, the multipath search using the sliding correlators, which take a rather long time, cannot follow the fluctuations of the delay profiles sometimes. Although fast multipath search may be possible by reducing the multipath search range and the number of times of the averaging, the reduction in the multipath search range will reduce the time diversity effect of the RAKE combining, and the reduction in the number of times of averaging the signal power will impair the accuracy of the RAKE combined multipath selection by the search fingers.
FIGS. 17A and 17B show as a related art (not as a prior art) a configuration of a receiver employing a matched filter in the DS-CDMA transmission system, which the assignee of the present invention proposed in Japanese patent application laid-open No. 10-190522 (laid open on Jul. 21, 1998) (that is, Japanese patent application No. 8-346025 filed on Dec. 25, 1996). In the configuration as shown in FIGS. 17A and 17B, a spread modulation signal received is amplified by the low noise amplifier 103, and then undergoes frequency conversion into the IF signal. The IF signal is fed to the AGC amplifier 107, and then to the square-law detector 108 that controls the amplifier 107 to compensate for the amplitude fluctuations caused by fading. Then, the amplifier output is fed to the quadrature detector 109 to undergo quadrature detection. The baseband signals output from the quadrature detector 109 are fed through the low-pass filters 110 and 111 to the A/D converters 112 and 113 to be converted into digital signals. A matched filter 131, which has pg taps, despreads using the output of a spreading code replica generator 132 the spread modulation signal converted into digital values, thereby dividing it into L timing signals, where L=pgxc3x97s, where s is the number of over samplings per chip. From among the L timings, W multipaths are selected, and the data demodulation is carried out by the differentially coherent detection, or by the coherent detection.
In this example, a scheme is used which carries out the absolute coherent detection demodulation using pilot symbols inserted into information symbols at fixed intervals in transmission frames. Channel estimators 116, which receive despread signals at L timings, respectively, estimate the channels using the pilot symbols, and supply the estimates to the multipliers 117. The multipliers 117 multiply the estimates by the outputs of the matched filter 131, thereby compensating for the channel fluctuations of the individual information symbols. On the other hand, average signal power measuring sections 134 measure the average received signal powers at L timings, and generate an average delay profile. A path selection timing detector 135 detects the maximum signal power of the profile obtained, and using the maximum signal power and a threshold decision gain, determines a threshold value for selecting the paths to be RAKE combined. Combined path selectors 133 select W greatest RAKE combined paths with signal powers higher than the threshold value. In this case, although the multipaths are selected from among the timings with large received power, the same multipath detected by the over sampling is excluded, and the next multipath is selected. The signals selected are combined by the adder 118 functioning as the RAKE combiner. The signal obtained by the RAKE combining is deinterleaved by the deinterleaver 122 and is decoded by the Viterbi decoder 123.
In the configuration employing the matched filter, despread signals are output at L timings per symbol period. This obviates the power measurement through the search fingers using the sliding correlators 119 as in the configuration as shown in FIGS. 16A and 16B. In addition, the update of the multipaths for the RAKE combining can be achieved quickly.
As described before, the delay profiles fluctuate and the number of multipath varies as the mobile station moves with respect to the base station. The configuration of FIGS. 17A and 17B, however, is arranged such that only the greatest W multipaths are combined. Thus, even if the number of the multipaths is greater than W, the ratio of the signal power to interference components and thermal noise cannot be improved by combining all the multipaths. Besides, when the number of the multipaths is less than W, the characteristic of the receiver will be degraded by combining multipath signals of low signal power, signals consisting of only noise components or interference components, or multipaths with very low received power.
FIGS. 18A and 18B show as a related art (not as a prior art) another configuration of a receiver employing a matched filter in the DS-CDMA transmission system, which the assignee of the present invention proposed in Japanese patent application No. 9-144167 (filed on Jun. 2, 1997), and published under the title of xe2x80x9cMatched Filter-based RAKE Combiner for Wideband DS-CDMA Mobile Radioxe2x80x9d (in IEICE TRANS. COMMUN. VOL. E81-B, NO. 7, JULY, 1998). In the configuration as shown in FIGS. 18A and 18B, the received spread modulation signal is amplified by the low noise amplifier 103, followed by the frequency modulation into the IF frequency by the circuits 104, 105 and 106. The IF signal is fed to the AGC amplifier 107, and the square-law detector 108 controls the amplifier 107 to compensate for the amplitude fluctuations caused by the fading. Subsequently, the amplifier output is fed to the quadrature detector 109 to undergo the quadrature detection. The baseband signals output from the quadrature detector 109 pass through the low-pass filters 110 and 111, and are converted into digital signals by the A/D converters 112 and 113. The matched filter 131, which has pg taps, despreads the signals converted into digital values, thereby outputting L(=pgxc3x97s) signals, where s is the number of over samplings per chip. From among the L timings, W multipaths are selected, and the data demodulation is carried out by the differentially coherent detection, or by the coherent detection.
In this example, Np pilot symbols are inserted into every Ns information symbol intervals in the transmission frames, and a scheme is used which carries out the absolute coherent detection demodulation using the pilot symbols. The channel estimators 116, which receive despread signals at L timings, respectively, estimate the channels using the pilot symbols, and supply the estimates to the multipliers 117. The multipliers 117 multiply the estimates by the output of the matched filter 131, thereby compensating for the channel fluctuations of the individual information symbols. The average signal power measuring sections 134 measure the average received signal powers at L timings, and generate an average delay profile. The average signal power measurement is carried out using the pilot symbols, for example. A minimum power detector 141 and a maximum power detector 142 selects, from the average received powers at L timings obtained by the average signal power measuring sections 134, minimum signal power and maximum signal power, respectively. A threshold value A controller 144 obtains a threshold value A using the minimum signal power detected, and a threshold value B controller 145 obtains a threshold value B using the maximum signal power detected. The threshold values A and B are obtained by multiplying the minimum signal power and the maximum signal power by different gains. A path selection timing detector 146 compares the outputs of the average signal power measuring sections at the L timings with the threshold values A and B. and detects timings providing the average signal powers equal to or greater than the threshold values A and B. Then, beginning from the timing providing the greatest signal power, the timings of the multipaths are sequentially detected. Specifically, excluding signals within a range of xc2x1k (k is a natural number) timings with respect to the multipath timings that are already selected, the path selection timing detector 146 sequentially detects the next multipath timings. The combined path selectors 133, receiving timings output from the detector 146 and the outputs of the multipliers 117, select the demodulator outputs at the multipath timings detected. The signals selected are combined by the RAKE combiner 118. The signal obtained by the RAKE combining is deinterleaved by the deinterleaver 122 and is decoded by the Viterbi decoder 123.
This configuration, which selects the multipaths to be RAKE combined with reference to the two threshold value, can combine only the signals with effective signal power, thereby reducing the effect of the signals consisting of noise and interference. In addition, even when the number of effective multipaths varies because of the fluctuations of the delay profiles, the multipaths satisfying the threshold values can be combined. The fixed threshold values in this configuration are effective to the delay profile of particular forms and particular fluctuations. The form and variation of the delay profile, however, are diversified in actual mobile communication environments, which will hinder the RAKE combining of the effective signals when the fluctuations of the delay profile cannot be followed, or will impair the characteristics of the receiver because of the rather large effect of the noise and interference components.
An object of the present invention is to provide a RAKE receiver capable of combining effective paths even when the number of multipaths varies because of the fluctuations of the delay profiles.
More specifically, the object of the present invention is to provide a RAKE receiver capable of carrying out effective RAKE combining by always selecting effective multipaths by following the dynamic fluctuations of the delay profile by adaptively controlling weighting factors such that MSE (Minimum Square Error) becomes minimum (MMSE) and by using these weighting factors.
Another object of the present invention is to provide a RAKE receiver which can reduce the convergence time of the MMSE by determining initial values of the weighting factors.
Still another object of the present invention is to provide a RAKE receiver capable of improving the characteristics of its receiving quality through time diversity effect by RAKE combining of the wideband DS-CDMA with high chip rate.
In order to achieve the above-described objects, according to the invention of claim 1, a RAKE receiver in a direct sequence CDMA transmission system for carrying out multiple access transmission by spreading information data into a wideband signal using a spreading code, comprises: a spreading code replica generator for generating spreading code replicas; a matched filter having a plurality of taps for despreading a received spread modulation signal by using the outputs of the spreading code replica generator; a weighting factor controller for controlling weighting factors corresponding to the outputs of the matched filter by using the respective despread signals fed from the matched filter and an output of an error signal generator in such a manner that the output of the error signal generator becomes minimum; multipliers for multiplying the respective despread signals fed from the matched filter by the corresponding weighting factors fed from the weighting controller; demodulators for demodulating individual signals fed from the multipliers; an adder for combining the signals fed from the demodulators; a data decision section for performing data decision with respect to the output of the adder; and the error signal generator for calculating a difference between the output of the adder and the output of the data decision section so as to generate an error signal.
According to the invention of claim 2, a RAKE receiver in a direct sequence CDMA transmission system for carrying out multiple access transmission by spreading information data into a wideband signal using a spreading code, comprises: a spreading code replica generator for generating spreading code replicas; a matched filter having a plurality of taps for despreading a received spread modulation signal by using the outputs of the spreading code replica generator; demodulators for demodulating individual despread signals fed from the matched filter; a weighting factor controller for controlling weighting factors corresponding to the respective outputs of the demodulators in such a manner that an output of an error signal generator becomes minimum; multipliers for multiplying the respective demodulated signals fed from the demodulators by the corresponding weighting factors fed from the weighting factor controller; an adder for combining the outputs of the multipliers; a data decision section for performing data decision with respect to the output of the adder; and the error signal generator for calculating a difference between the output of the adder and the output of the data decision section so as to generate an error signal.
According to the invention of claim 3, a RAKE receiver as claimed in claim 1 or claim 2 comprises:
a signal power measuring section for measuring average received signal powers of the respective outputs of the matched filter; a minimum power detector for detecting a minimum signal power based on the outputs of the average signal power measuring section; a threshold value controller A for determining and outputting a threshold value A for selecting signals to be combined in the adder based on the output of the minimum power detector; a path selection detector for comparing the outputs of the average signal power measuring section with the output of the threshold value controller, so as to detect signals, received powers of which are equal to or greater than the threshold value A; and a combined path selector for selecting signals corresponding to the signals detected by the path selection detector from among the despread signals fed from the matched filter.
According to the invention of claim 4, in a RAKE receiver as claimed in claim 3, the weighting factor controller controls only the weighting factors corresponding to the signals detected by the path selection timing detector when the weighting factor controller determines the weighting factors.
According to the invention of claim 5, a RAKE receiver as claimed in claim 1 or claim 2 comprises:
a signal power measuring section for measuring average received signal powers of the respective outputs of the matched filter; a minimum power detector for detecting a minimum signal power based on the outputs of the average signal power measuring section; a maximum power detector for detecting a maximum signal power based on the outputs of the average signal power measuring section; a threshold value controller A for determining and outputting a threshold value A for setting initial values of the weighting controller based on the output of the minimum power detector; a threshold value controller B for determining and outputting a threshold value B for setting initial values of the weighting controller based on the output of the maximum power detector; an effective path detector for comparing the outputs of the average signal power measuring section with the output of the threshold value controller A and with the output of the threshold value controller B so as to detect signals, signal powers of which are equal to or greater than the threshold value A and the threshold value B; and an initial weighting factor setting section for placing initial values of the weighting factors corresponding to the signals detected by the effective path detector at a, where 1 xc3x85xc3x9c a greater than 0, and for placing initial values of the weighting factors corresponding to the remaining signals at zero.
According to the invention of claim 6, a RAKE receiver as claimed in claim 3 or claim 4 comprises: a maximum power detector for detecting a maximum signal power based on the outputs of the average signal power measuring section; a threshold value controller B for determining and outputting a threshold value B for setting initial values of the weighting controller based on the output of the maximum power detector; an effective path detector for comparing the outputs of the average signal power measuring section with the output of the threshold value controller A and with the output of the threshold value controller B so as to detect signals, signal powers of which are equal to or greater than the threshold value A and the threshold value B; and an initial weighting factor setting section for placing initial values of the weighting factors corresponding to the signals detected by the effective path detector at a, where 1 xc3x85xc3x9c a greater than 0, and for placing initial values of the weighting factors corresponding to the remaining signals at zero.
According to the invention of claim 7, in a RAKE receiver as claimed in claim 5 or claim 6, the weighting factor controller sets the values determined by the initial weighting factor setting section as the initial values of the weighting factors corresponding to the signals in setting the initial values of the weighting factors.
According to the invention of claim 8, in a RAKE receiver as claimed in any one of claims 1 to 7, the weighting factor controller, in controlling the weighting factors, unconditionally places at zero the weighting factors at xc2x1k timings before and after a timing of the matched filter providing the maximum weighting factor, where k is a natural number, and sequentially determines the weighting factors by detecting a timing providing the second maximum weighting factor.
The RAKE receiver in accordance with the present invention can reduce the effect of signals consisting of only noise or interference components because it performs the RAKE combining after weighting all the signals a matched filter despreads at the entire timings by the weighting factors that are subjected to MMSE control.
Furthermore, according to the present invention, even if the number of the multipaths varies because of the fluctuations in the delay profile, only effective paths can be combined. This has an advantage of being able to RAKE combine only the effective multipaths following the fluctuations in the delay profile.
Moreover, according to the present invention, because the average received powers of the despread signals are measured at all timings, and threshold value or values are set from the measured results, the effect of the noise or interference can be further reduce. Besides, the convergence time of the MMSE can be reduced because the initial values of the weighting factors based on the MMSE control can be determined in advance.
In addition, according to the present invention, improvement in the characteristics of receiving quality can be implemented by the time diversity effect of the RAKE combining in a high chip rate, wideband DS-CDMA.